1. Field of the Invention
The present invention relates to power converters, and more particularly, to synchronous regulation circuit of power converters.
2. Description of Related Art
An offline power converter includes a power transformer to provide isolation from AC line input to the output of the power converter for safety. In recent developments, using soft switching topologies in the primary side of the transformer and applying the synchronous rectifier in the secondary side of the transformer, high power conversion efficiency for power converters can be achieved. Among them, the full-bridge quasi-resonant ZVS (zero voltage switching) techniques are taught in “Constant Frequency Resonant Power Converter with Zero Voltage Switching” by Henze et al, U.S. Pat. No. 4,855,888; “Soft-switching PWM Converters” by Hua et al, U.S. Pat. No. 5,442,540; and “Zero Switching Power Converter Operable as Asymmetrical Full-bridge Converter” by Yang, et al, U.S. Pat. No. 6,744,649. The drawback of foregoing power converters is that the pulse width of the switching signal is too short to achieve soft switching operation at light loads. Besides, the insufficient circular power causes hard switching and low efficiency. The synchronous rectifier may be equipped at the secondary side of the transformer to reduce the power loss of the diode. Detail operations of the synchronous rectifying can be referred in a prior art titled “Control Circuit Associated with Saturable Inductor Operated as Synchronous Rectifier Forward Power Converter” by Yang, U.S. Pat. No. 7,173,835. However, additional devices such as saturable inductors and current-sense resistor incur inevitable power consumption.